1. Technical Field of the Invention
The present invention relates to a light shielding blade material for an optical apparatus which requires LIGHT shielding properties for use in a shutter blade or diaphragm blade of a focal plane shutter or lens shutter of a camera, and a manufacturing method of the material. For details, the present invention relates to a light shielding blade material formed of a composite resin reinforced by carbon fibers.
2. Related Art
A plate material formed of carbon-fiber reinforced plastics (CFRP) has frequently been used as a shutter blade of a focal plane shutter. The blade constituted of CFRP is light in weight and high in bend rigidity, and has little corrugation of the blade during run and immediately after stop even at a high shutter speed exceeding 1/8000 second. Above all, in the CFRP plate suitable for the high-speed shutter, a carbon fiber aligned as a reinforcing fiber in one direction is used, and an epoxy resin is used as a matrix resin. The CFRP plate material is manufactured by superposing a plurality of prepreg sheets as precursors so that fiber directions cross at right angles to one another, and pressing, heating, and curing the whole laminate plate. The CFRP plate manufactured in this manner is cut in practical shapes of light shielding blades. The blade is generally pressed and punched. The light shielding blade using such CFRP plate is disclosed, for example, in Japanese Patent Application Laid-Open Nos. 6-160938, 10-301158, 11-194393, and 2000-75353.
However, the CFRP plate has a tentative light shielding property because of a black carbon fiber, but to secure a complete light shielding property required for a shutter blade, it is necessary to add carbon black to a matrix resin. A prepreg sheet in which the carbon black is mixed into the matrix resin is a custom-made item, cost is therefore high, and need for production in a larger amount arises. Therefore, a high-cost and large-amount stock is required, and this causes a cost increase of the light shielding blade formed of CFRP.
There has been a demand for a light shielding blade material for an optical apparatus in which a commercially available carbon-black-free prepreg is used without adding the carbon black to a prepreg as a raw material and which has a practical price. One example of the material is disclosed in the Japanese Patent Application Laid-Open No. 10-301158. According to this, in a light shielding blade formed of a carbon-fiber reinforced resin formed by laminating a plurality of reinforced resin sheets comprised of a matrix resin containing carbon fiber aligned in one direction so that fiber directions cross at right angles to one another, a homogeneous resin layer is disposed as an intermediate layer between the reinforced resin sheets. The carbon black is mixed in the resin layer which is the intermediate layer. Concretely, the prepreg sheet of the matrix resin such as the epoxy resin is superposed upon the resin layer of polyurethane in which the carbon black is mixed, and heated/pressurized/molded to obtain a light shielding blade plate material.
However, the material of the matrix resin is different from that of the intermediate resin, and the CFRP plate therefore has a possibility that an interface is peeled or deformed by influences of temperature, and does not necessarily have a stable shape. Moreover, the carbon black mixed in the intermediate resin layer is not dispersed into the prepreg and is confined substantially within the intermediate urethane layer, and therefore unevenness is sometimes generated in-the shielding property.